1. Field of the Invention
An object of the present invention is a method for the testing of electronic components taking into account the drift in the mean of the output response. It can be used especially in the field of the testing of semiconductor electronic components, especially components such as integrated circuits. In the prior art, there is a known method for testing electronic components that reduces the unit testing time for each of the components. The value of the invention is that it proposes a test method that, first of all, reduces the unit testing time and, secondly, can be used to monitor drifts in responses of these tests, these drifts being caused, for example, by an as yet tolerable but possibly disturbing modification of the tested components.
A component to be tested is therefore subjected to a series of unit tests for the testing of each of its functions. A unit test of a function generally comprises the following elementary steps:
at an initial date D0, a terminal of the component, considered to be an input terminal with respect to the test, is subjected to an electrical potential Ve,
there is a wait, for a specified time period, for the appearance and stabilization of a response at a second terminal of this component, this second terminal being considered to be an output terminal with respect to this test,
at a nominal measurement date Dm0 at the end of the time period (Dm0xe2x88x92D0), this response is measured. In one example, this response is a value Vs of a potential at this output terminal.
Then, to ascertain that the component has given an acceptable or unacceptable response to this unit test,
the measured response is compared with fixed acceptance criteria. Namely, in this example, the value of the potential VS is compared with lower and upper acceptance limits. These limits are generally given in manufacturing specifications.
Since the electronic component has a known structure, the time period at the end of which it is possible to observe an expected response with respect to the test is determined theoretically. This theoretical time period (Dmthxe2x88x92D0) is for example computed by the designer of the electronic component. Furthermore, in order to avoid having an excessively high failure rate for this test, a safety margin is taken into account by generally choosing the nominal measurement date Dm0 which is later than the theoretical measurement date Dmth of this test.
Then, the nominal measurement date Dm0 is used as a measurement date for all the components to be tested. This choice, which ensures a response in conformity with the test, has the consequence of considerably increasing the total execution time of a series of tests.
2. Description of the Prior Art
The document WO-A-97/45748 describes a known test method used to reduce the duration of each unit test of a series of tests. To this end, the test method comprises:
a first xe2x80x9clearning phasexe2x80x9d carried out on a population of acceptable components.
The nominal measurement date Dm0 is used to perform tests on the first components of the batch. Only the components that have given an appropriate response (within the acceptance limits) are incorporated into the population of acceptable components of this batch, this population constituting the learning population. Then, as far as possible, a measurement date Dmi earlier than the nominal measurement date Dm0 is determined. The tests performed with this measurement date Dmi must give a still acceptable response with respect to the acceptance limits,
a second xe2x80x9capplication phasexe2x80x9d during which all the other components of the batch are tested, by using the earliest measurement date Dmi determined beforehand.
To determine the earliest measurement date Dmi, the elementary steps of the test defined here above are reiterated on each of the components of the learning population, the measurement date applied being reduced at each iteration, either dichotomously or step by step. To choose the measurement date Dmi among the intermediate measurement dates tested, Dmi-TEST1, Dmi-TEST2, the following are compared by means of a criterion:
a statistical image of the responses obtained with at least one tested intermediate measurement date, for example Dmi-TEST1, with
a statistical image of the responses of this very same learning population, obtained at the nominal measurement date Dm0.
A statistical image comprises all the responses, of the VS type, obtained during the test and especially the computation, on the basis of these values, of the mean M and the standard deviation S, and of all the other statistical values as a function of this mean M and this standard deviation S.
For example, for each statistical image, a statistical value referenced CP is obtained. This value is equal to a ratio between a difference of acceptance limits and the standard deviation S of this statistical image. The difference between the acceptance limits is generally given by a manufacturer""s tolerance.
Another statistical value can also be defined to characterize this statistical image. This other value referenced CPK is then equal to the ratio between an absolute value of a difference between the mean and an acceptance limit and the standard deviation S of this statistical image.
To compare the statistical images with one another, their respective statistical values CP or CPK are compared. The earliest possible measurement date Dmi is chosen from the tested intermediate measurement dates such that its corresponding statistical value CPi obtained with the measurement date Dmi remains within a specified proportion of the appreciation criterion CPo, where CPo is the statistical value characterizing the statistical image obtained with the nominal measurement date Dm0.
In a first example, if a wide range of responses is observed for a tested intermediate date Dmi-TEST1 without any predominance of any value, it means that the behavior of the electronic component under this test is not reliably reproducible on this date. According to the method of the invention, the standard deviation Si-TEST1 of the statistical image is great and therefore the corresponding value CPi-TEST1 will be low, and will certainly no longer be within the set criterion of the specified proportion of CPo. Then, this measurement date Dmi-TEST1 will no longer be chosen as the earliest possible measurement date Dmi, and Dmi will necessarily be greater than this tested date Dmi-TEST.
The method of determining the earliest measurement date Dmi may include another acceptance condition, for example a condition by which this date is necessarily greater than a theoretical minimum measurement date Dmmin.
The solution proposed in this document WO 97/45748 is problematic because the learning phase preliminary to the execution of the application phase is excessively lengthy. Furthermore, the earliest possible measurement date determined during this learning phase may result in a greater discard rate for the electronic components tested because it may cause these components to be unjustifiably rejected from the good quality production. Indeed, since the learning phase is long, it cannot be carried out too frequently, and then an inappropriate measurement date may be used to test a part of the batch.
From the teaching of the document FR 99 09055, there is a known method also comprising a learning phase and an application phase; and further adding an adjusting phase.
However, this method provides that the learning phase used to define the earliest possible measurement date Dmi is obtained by comparing:
a statistical image between the responses of the learning population obtained with the nominal measurement Dm0 and
a cumulated statistical image, this image corresponding to the image of all the responses obtained on this same population with the tested intermediate measurement date, for example Dmi-TEST1, and with the responses obtained on this population on the nominal measurement date Dm0.
To compare the two statistical images with each other, their respective statistical values CPK are compared in the same way. The earliest possible measurement date Dmi is chosen such that the statistical value CPKtotal:0+i-TEST1 remains within a certain proportion of the criterion of appreciation Cpko where Cpko is the statistical value characterizing the statistical image obtained with the nominal measurement date Dm0.
Furthermore, this method provides that the application phase will be independent of the learning phase which is performed on a learning population independent of the components of the batch to be tested during the application phase. Thus, the learning phase is achieved independently and only a short adjusting phase is carried out when starting a series of tests to be performed on a batch of components. During this adjusting phase, the earliest measurement date Dmi is validated and possibly adjusted.
These adjusting phases occur regularly. For example they occur at a certain frequency depending on the given number of the tested parts of the batch. They ensure that the measurement date is always optimized with respect to the duration of the test and optimized with respect to the profitability of this tested output in taking account of the natural variability of the characteristics of production.
During this adjusting phase, a population of good quality components is considered. And just as in the learning phase, the earliest measurement date Dmi is verified and possibly modified by comparing:
a statistical image of the responses of the population of good quality components obtained with the nominal measurement date Dm0 with
a cumulated statistical image, this image corresponding to the image of all the responses obtained on this same population with the measurement date Dmi as well as the responses obtained on this population at the nominal measurement date Dm0 or with a modified measurement Dmmodif as well as the responses obtained on this population at the nominal measurement date Dm0.
As the case may be, it is chosen to apply a modified measurement date Dmmodif, Dmmodif being greater than or equal to the measurement date Dmi if the comparison between (a) the cumulated statistical image, comprising inter alia the responses obtained on the measurement date Dmi and (b) the statistical image of this same population obtained with a nominal measurement date Dm0 is not acceptable with respect to a criterion of appreciation.
This approach too raises a problem. Indeed, even if the method comprises an adjusting phase to take account of the intrinsic variability of the production of components, this method tolerates the presence of drift within this production and does not note this drift. According to this test method, the variability of the responses and their drift, if any, are not considered because the statistical images representing these data are compared by means of a statistical value: CPK. Now this statistical value CPK is computed by incorporating lower and upper acceptance limits, and these limits are determined as a function not of the method of manufacture of the components but of tolerable limits with respect to the quality of the manufactured components, namely the specifications of these components.
This method therefore results in the determining of statistical values CPK in the range of a thousand when they are values of response of a statistical image obtained for a measurement date close to the nominal measurement date Dm0. However, since the criterion of appreciation of this statistical value is a comparison with a criterion of acceptance in the range of 3, the mode of determining the earliest possible measurement date Dmi leads to the choice of a measurement date Dmi very close to the minimum measurement date Dmmin. Consequently, the test, with this method, will accept components that have given a response very close to one of the acceptance limits. This however, can be a sign of a drift in the manufacture of these components to be detected.
And even if the adjusting phase corrects the choice of an excessively early measurement date Dmi, the adjusting phase then becomes almost as long as the learning phase, and this too is a drawback. Indeed, this test method makes it necessary to almost routinely increase this measurement date Dmi during the adjusting phase because the date defined during the learning phase is unsuited to the output to be tested.
It is an object of the invention to overcome this problem by making a faster detection of the signs of drift in the manufacture of the components. The invention makes it possible especially to limit the search for the earliest measurement date Dmi to the determining of a measurement date for which the drift of the mean is controlled. Hence, the earliest possible measurement date Dmi determined according to the method of the invention ensures that reliable responses are obtained to the test. However, the earliest measurement date Dmi determined according to this method may be later than the one which would have been obtained by using another test method also seeking to reduce the total duration of the testing time.
Indeed, an object of the invention is a method for the testing of electronic components that considers the statistical images of the population of components by representing them by statistical values which, first of all, do not take into account these acceptance limits on the output response. Furthermore, the method according to the invention compares the statistical images with one another, by considering statistical values that have at least one criterion of acceptance that is independent of the acceptance limits. The comparison thus made is far more strict and tolerates less variability of the responses to the test. The earliest measurement date defined according to this method guarantees greater reliability and reduces the need for this date to be corrected far too frequently.
An object of the invention is a method for testing electronic components, wherein
a terminal of a component is subjected to an electrical potential at an initial date,
a response set up at a terminal of this component is measured on a measurement date,
this response is compared with acceptance limits, for the acceptance or rejection of the component as a function of this comparison,
the measurement date is defined by means of a criterion applied to compare an intermediate statistical image representing at least responses obtained at a tested intermediate measurement date with a nominal statistical image representing responses obtained at a nominal measurement date, the responses being obtained from a population of good quality components,
the measurement date chosen is the earliest possible intermediate measurement date tested, wherein:
the criterion applied to compare the statistical images with one another comprises an assessment of a range of the responses obtained for each of the statistical images.